1. Field of the Invention
The present invention relates to a system with a digital signal processor, such as a computer, having full duplex speakerphone functionality.
2. Description of the Related Art
Systems capable of handling voice communication applications such as audio conferencing, teleconferencing, or telephony are increasingly integrating full duplex speakerphone functionality. In a full duplex speakerphone mode of a system, the speaker and microphone of a speakerphone are acoustically coupled such that sound waves from the speaker travel to the microphone. In order to prevent acoustic feedback due to sound waves traveling from the speaker to the microphone, acoustic coupling is reduced between the speaker and the microphone. This reduction is termed acoustic coupling loss. Acoustic coupling loss between a loudspeaker and a microphone of a speakerphone requires both a smooth loudspeaker frequency response and a smooth microphone frequency response.
Systems having speakerphone functionality such as certain portable computers have maintained sound reflections which degrade full duplex speakerphone performance by injecting frequency anomalies into a microphone response resulting in an irregular microphone response. For example, in a portable computer, sound reflections giving rise to frequency anomalies may result from numerous conditions such as the polar pattern of the microphone, the internal acoustic resonance of the microphone, or a near-field microphone effect. For a speakerphone user, these frequency anomalies are manifested as an unstable echo whereby the user hears his or her own voice continuously amplified. These anomalies inject acoustic coupling gain into a microphone response thereby increasing the acoustic coupling loss necessary to maintain a smooth microphone response. An acoustic echo canceler of a digital signal processor (DSP) has provided a limited amount of acoustic coupling loss to a system having sound reflections. This amount of coupling loss, however, has been insufficient to offset significant amounts of acoustic coupling gain caused by frequency anomalies.
Briefly, according to the present invention, a speakerphone uses a filter set or network to provide acoustic coupling loss at a microphone to offset acoustic coupling gain produced by sound reflections resulting from various conditions in a system. The filter network pre-processes an acoustic signal before the signal is provided to a digital signal processor to sufficiently eliminate acoustic coupling gain which the digital signal processor is unable to cancel or offset. The filter network is used to detect when the acoustic coupling gain in the acoustic signal at the microphone exceeds the maximum amount of acoustic coupling loss supplied by the digital signal processor. Alternatively, if acoustic coupling gain is desirable at the frequency location of the anomaly caused by the sound reflections, the filter network is used to boost the acoustic coupling gain produced by the anomaly. Thus, the present invention uses equalization at the microphone to boost or attenuate acoustic coupling gain caused by frequency anomalies before acoustic echo cancellation by the digital signal processor.